Metered acid acceleration of hydrogen generation using seawater as a reactant
An underwater hydrogen generator can include a watertight reaction housing enclosing a metering chamber. The metering chamber can have an upper portion that terminates at a piston opening, and a lower portion that merges into a funnel, which can further terminate at a metering opening. The metering chamber can be filled with an acid accelerator, and the watertight reaction void can be partially filled with NaBH4 in solution. The generator can further include a seawater float valve that can be in fluid communication between the external environment, the metering chamber and the void defined by the reaction housing. The float valve, metering chamber and reaction housing can cooperate to generate hydrogen when said generator is submerged, by allowing seawater to contact both the acid accelerator and the NaBH4. The size of the metering opening can determine the rate at which acid accelerator is added to the NaBH4 solution.
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The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone (619) 553-5118; email: email@example.com, referencing NC 103552.FIELD OF THE INVENTION
The present invention pertains generally to methods and apparatus for generating hydrogen. More particularly, the invention describes a method and apparatus to generate hydrogen gas, using seawater as a reactant in the presence of acid accelerators, rather than transition metal catalysts. The invention is particularly, but not exclusively, useful as an undersea method and apparatus for generating hydrogen while submerged, which can meter the acid accelerator in seawater into the hydrogen-containing material in seawater, in order to provide a sustained, controlled source of hydrogen for various underwater applications.BACKGROUND OF THE INVENTION
As known in the prior art, metal hydrides are particularly useful for the generation of hydrogen gas whenever the use of compressed gas is inconvenient or infeasible due to storage considerations. The metal hydrides lead in hydrogen-generating capacity per unit weight. Sodium borohydride, for example, can react slowly with water, but more quickly in the presence of a transition metal catalyst, to liberate four moles of hydrogen gas per mole of the compound, or 2.4 liters H2 gas per grams of compound (L/g) at room temperature, as shown in Equation (1):
- Transition Metal Acc.
NaBH4+2H2O(l)→NaBO2+4H2(g); ΔH=−72.5 kcal/mole Eq. (1)
As can be seen from the above, transition metals can be used as accelerators for generation of hydrogen in many applications, and particularly in conjunction with sodium borohydride. In addition to its large hydrogen generating capability, sodium borohydride can have a number of advantages over the metals and other metal hydrides, in that it is readily available in granular form at a moderate price, it is stable for months in alkaline solution, its solubility in water is high, and it is relatively safe to handle, especially when it can be transported and stored in dry form. But for other applications, in particular those that envision that use of seawater for interacting with metal hydrides to generate hydrogen, transition metals can be ineffective as accelerators for reactions that involve sodium borohydride.
- Transition Metal Acc.
One possible solution could be to use acid accelerators, vice transition metal catalysts, when generating hydrogen from metal hydrides and water. Prior studies are known in the art for describing the effect of acid accelerators and catalytic accelerators on the evolution of hydrogen gas from sodium borohydride, but in pure (DI) water. Other studies in the prior art can describe the evolution of hydrogen gas from lithium borohydride using steam as an accelerator. There is also prior art that can describe the generation of hydrogen gas from metal hydrides and seawater, as opposed to pure water, using acid accelerator NaHSO4. But in the prior art, the disclosed methodology refers to an open system and can be only thirty to fifty percent efficient. This can be an ineffective efficiency level for many applications, especially for applications which are space or weight-limited, and would require that the amount of acid accelerator used be kept to a minimum.
In view of the above, it can be an object of the present invention to provide methods and apparatus for generating hydrogen that use acid accelerators, instead of transition metal accelerators. Another object of the present invention can be to provide methods and apparatus for generating hydrogen that use seawater as a reactant in hydrogen generation, which can allow for subsea hydrogen generation without having to transport DI water reactant to the reaction site. Another object of the present invention can be to provide methods and apparatus for generating hydrogen with starting materials that are stable for long periods of time, for ease of storage. Still another object of the present invention can be to provide methods and apparatus for generating hydrogen that are sufficiently efficient for undersea naval applications. Yet another object of the present invention can be to provide methods and apparatus for generating hydrogen that can generate sufficient hydrogen yield to function as part of a hydrogen source for an underwater fuel cell. Yet another object of the present invention can be to provide methods and apparatus for generating hydrogen using seawater that can be easy to implement in a cost-effective manner.SUMMARY OF THE INVENTION
An underwater hydrogen generator in accordance with several embodiments of the present invention can include a metering casing defining a metering chamber and a watertight reaction housing enclosing the metering casing so that the casing and housing are concentric. The watertight reaction housing and casing can cooperate to define an annular reaction void. The metering casing can have an upper portion that terminates at a piston opening, and a lower portion that merges into a funnel, which can further terminate at a metering opening. The metering chamber can be filled with an acid accelerator, and the watertight reaction void can be partially filled with the sodium borohydride (NaBH4) in solution. The underwater hydrogen generator can further include a seawater float valve in fluid communication between the external environment and the metering chamber, as well as the reaction void.
When the generator is submerged, the float valve, metering casing and reaction housing can cooperate to generate hydrogen by allowing seawater to flow into metering chamber and reaction void to contact both the acid accelerator and the NaBH4. A weighted piston can be inserted into the piston opening, to accomplish a syringe-like action and urge the acid accelerator/seawater combination through the metering opening when the generator is submerged. The size of the metering opening can determine the rate at which acid accelerator is added to the NaBH4 solution. The accelerator can be B2O3 or NaHSO4, although other accelerators could be used. A hydrogen output port can be formed in the reaction housing, and the generated hydrogen can be routed through an exit port on the generator and routed into a fuel cell that can be in fluid communication with the hydrogen exit port.
The novel features of the present invention will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similarly-referenced characters refer to similarly-referenced parts, and in which:
Referring initially to
Referring now to
BH4−+2HSO4−+2OH−→BO2−+2SO42−+4H2(g) Eq. (2)
Measurements of NaHSO4 in seawater using the ratios g NaHSO4:g NaBH4=1:1 and g H2O:g NaBH4=20:1 can be shown in
Even with the 1.33:1 ratio, the proportions are hardly stoichiometric. Moles of reactants used in the present experiment are given above for BH4− and HSO4− in Eq. (2). There can be a considerable stoichiometric shortfall of HSO4−.
Referring now to
Referring now to
The acid accelerator proposed for the present invention in some embodiments can be boric oxide, B2O3, which, unlike the transition metal catalysts, takes part in the reaction:
NaBH4+½B2O3+2H2O(l)→½Na2B4O7↓+4H2(g)ΔH=−60.7 kcal/mole Eq. (3)
The reaction product disodium tetraborate, commonly known as borax, can precipitate out and in some embodiments can be recovered from the reaction mixture. Borax has many uses in the prior art, i.e., detergent booster and multi-purpose household cleaner. Moles of reactants for NaBH4 and B2O3 that can be used for the present invention according to several embodiments can be given above in Eq. (3). For the mole ratio cited in Eq. (3), there can be a slight stoichiometric excess of B2O3, which can ensure that the reaction will go to completion.
Referring now to
The above embodiments demonstrate a relatively brief time to completion using boric acid (B2O3) as the accelerator. For several embodiments, it may be advantageous to slow down the reaction. In such cases, B2O3 can be made to liberate hydrogen more slowly by dripping (metering) the more soluble NaBH4 into the less soluble B2O3 in a controlled manner. The metering can be done in a variety of ways; such as a manual drip (gravity or otherwise) using an addition funnel, or an automatic delivery method using a peristaltic pump. These methods can be described more fully below, and other delivery methods could certainly be used.
Referring now to
The analogous reaction of B2O3 in seawater with an addition funnel can be seen in
Referring now to
Referring now to
From the above, it can be inferred that the difference in the order of metering for the NaBH4/B2O3 system and the NaHSO4/NaBH4 system previously discussed can be important. For some embodiments, delivery of NaBH4 into accelerator B2O3 can be optimal, since NaBH4 is extremely soluble in H2O (55 g/100 g H2O at 20° C.) while B2O3 has limited solubility. However, unlike the transition metal catalysts, the acid accelerators participate in the hydrogen generation reaction so that B2O3 will become more soluble as the reaction proceeds. For other embodiments, the NaHSO4/NaBH4 system, delivery of accelerator NaHSO4 into NaBH4 can be recommended since the NaBH4 can be kept isolated, and cannot be given the opportunity to form H2 bubbles in the delivery line. NaHSO4 is relatively soluble in H2O so that all the NaHSO4 will be dissolved in the solution to be delivered prior to delivery. Since the solubility of NaHSO4 is 2.57 g/9 ml at 25° C., the ratio g seawater:g NaBH4 can be reduced from 20:1 to 9:1. This lower limit makes available an amount of NaHSO4 that is still larger than the 2.3 g that is required for each 1.5 g NaBH4.
Referring now to
The reaction time in seawater is faster than the reaction time in DI water as can be seen by comparing the bulk reaction times for seawater (1.35 minutes, See
Referring now to
Referring now to
Table 4 above shows that for seawater using metering of accelerator, the peak temperature can be decreased almost 20 degrees and the reaction time can be made longer by a factor of 17 above bulk addition. Metering NaBH4/seawater into accelerator B2O3/seawater (or NaHSO4/seawater into NaBH4/seawater in some embodiments) can allow the reaction time and temperature to be tailored to the requirements of a particular application. But in all cases, the reaction goes to completion in seawater, which can meet the efficiency needs of the Department of the Navy for underwater fuel cells.
Referring now to
As shown in
As accelerator 40 passes into reaction void 23, it combines with the seawater/NaBH4 solution, and hydrogen gas is generated, as indicated by arrows 46. The hydrogen gas rises and passes through exit port 48, where it is used by an H2/O2 fuel cell, indicated generally in
Given the solubility of NaHSO4 and NaBH4 as described earlier, these constituents could be used in the metering chamber 22 depending on which acid accelerator 40 can be used, NaHSO4 or B2O3 respectively. As designed for a 20:1 g H2O per g NaBH4 ratio, the metering chamber 22 can contain one-fourth of the reactant volume and the reaction void 23 can contain the remainder of the reactants. A water ratio of 5:1 water to reactant will be used in the metering chamber 22; the remainder of the water for the reaction can be added to the reaction void 23. However, a reduction in volume of the reactants and consequently chamber volume can be achieved by lowering this ratio directly. For example, if a 10:1 g H2O per g NaBH4 ratio is desired for the same amount of NaBH4, a water ratio of 2.5:1 water to reactant will be used in the metering chamber; 3 parts seawater containing the remaining reactant 42 can be added to the reaction void 23. The use of less water can allow the size and weight of the apparatus 10 to be reduced.
Seawater 38 can be added by various means, including but not limited to a peristaltic or other type of pump (not shown in the Figures) or by taking advantage of the ambient pressure around the generator 10, when generator 10 is submerged at some minimum depth (as shown in
For operation, water can be added by either flooding the metering chamber 22 or pumping seawater into the metering chamber 22. It can important to note that the reaction void 23 must not be filled completely. A free volume of gas above the reaction level 52 is required such that the reaction volume can expand due to release of hydrogen gas. The total free volume above the reaction level will dictate the maximum flow rate. This can be necessary to prevent reactants or foam from being pushed out of the top of the apparatus 10. A pressure relief valve 54 in the metering chamber 22 can prevent high pressure gas from causing the generator 10 to meter too quickly. As mentioned above, a dissolving plug 44 may be added in metering opening 32 to prevent the metering chamber 22 from metering accelerator before the chamber 22 is filled.
In some cases (such as when NaBH4 exists in metering casing 20) a small amount of gas can be formed in casing 20. The gas reaction cannot be mitigated by pressure, thus additional pressure can lead to a differential pressure in the chambers. This will increase delivery rates. If there were no valve 54, this would occur. In the case where there is no excess gas in casing 20 the small travel space in such a valve would prohibit the fluid from escaping beyond reason (which is what would happen if the piston had a hole). Because NaBH4 would react with seawater to some degree, the valve 54 can keep the differential between metering casing 20 and reaction housing 18 at a controlled level. Reaction housing 18 does not necessarily need a relief valve, but increased pressure in 18 as a whole makes the product much less laden with water vapor. Because the reaction and device are agnostic to temperature (no thermal runaway), allowing a high temperature in the system is acceptable to some degree.
One good way of limiting water vapor in the hydrogen product can be increasing internal pressure since water vapor partial pressure is dependent only on temperature. Thus a higher chamber pressure will intentionally limit the water vapor output in terms of percentage of total gas out. Lastly, it is important to note that if the chamber exists with 1 atmosphere (atm) outside the chamber and there is no valve in reaction housing 18, the reaction will occur at 1 atm and will allow water to naturally limit the temperature of the reaction through its heat of vaporization. Thus the system cannot increase beyond 100° C., and likely will not exceed ˜95° C. regardless of rate of reaction. Obviously, the reaction rate is limited by “foam” volume. This foam has been shown to be the hydrogen gas forming in solution. A higher pressure chamber 22 can also decrease the size of the bubbles and thus the level of foam.
As mentioned above, water can be cut off from the metering chamber as mentioned when the weighted piston reaches its highest point, analogous to when a syringe is fully extended. Water can be cut off from entering the lower chamber by either a diverter valve, an automatic float valve, or an electric controlled valve. If a pump is used, that pump will be turned off once the chamber is full unless the pump will be used to circulate fresh seawater as a cooling method in the reaction void through cooling coils (not shown). Once the plug in the funnel has dissolved, or once the diastolic pump is activated, the metering through the funnel can begin. The weighted piston will continue to provide consistent head to either method for a more uniform flow.
The hydrogen generation reaction of the present invention according to several embodiments can continue to completion and hydrogen gas is collected from the upper relief valve. To reduce foam or reactant level, a high pressure relief valve may be used. It is important to note that the metering chamber 22 can be automatically pressure compensated when higher pressures are used as long as the top side of the piston is open to the reaction void. To achieve a faster reaction with a smaller chamber, or more consistent gas temperature, active cooling could be employed. It is important to note that higher temperatures will increase water vapor in the product gas. Water vapor can be additionally reduced by operating at a higher pressure in the module.
For setup and storage, placement of the chemicals could be accomplished during module assembly of upper part 12 and lower part 14 with retaining rings 16. Since the module will be stored without water, it can remain in an assembled and “loaded” state for long periods of time. The NaBH4 and acid accelerator disclosed above are stable under a wide range of temperature, pressure and humidity conditions.
The systems and methods of the present invention can allow for the use of acid accelerator B2O3 for hydrogen generation using seawater as a reactant. The fact that B2O3 is not poisoned by seawater like the traditional transition metal catalysts used for hydrogen generation can be extremely advantageous for the present invention according to several embodiments. The use of metering to deliver the accelerator or the NaBH4 can also be advantageous as well as the fact that the methods of the present invention are order-agnostic. That is, the acid accelerator 40 (NaHSO4) can be placed in the metering chamber 22 and can be metered into the NaBH4 reactant 42 in the annular reaction void 23, or vice versa, i.e., the NaBH4 reactant 42 can be placed in the metering chamber and metered into the acid accelerator 40 in the annular reaction void 23. Still further, the reaction of sodium borohydride with B2O3, as opposed to transition metal catalysts, can result in a lower pH reaction product, tetraborate Na2B4O7, which can be less corrosive than metaborate NaBO2.
Referring now to
The use of the terms “a” and “an” and “the” and similar references in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
1. A method for underwater generation of hydrogen, said method comprising the steps of:
- A) providing one of NaBH4 or an acid accelerator in a watertight reaction housing;
- B) affording the other of said NaBH4 or said acid accelerator in a metering casing so that said acid accelerator and said NaBH4 are in fluid isolation, said watertight reaction housing being concentric to said metering casing; and,
- C) selectively metering said acid accelerator into said NaBH4 with a seawater valve, said seawater valve establishing selective seawater fluid communication with said NaBH4 and said accelerator when said watertight reaction housing is submerged.
2. The method of claim 1, wherein said metering casing has an upper portion that terminates at a piston opening, and a lower portion that merges into a funnel, said funnel terminating at a metering opening, and further wherein said step C) is selectively accomplished by selecting the size of said metering opening.
3. The method of claim 1, wherein said acid accelerator is B2O3.
4. The method of claim 1, wherein said acid accelerator is NaHSO4.
|4198475||April 15, 1980||Zaromb|
- Becker-Glad et al. “Acid acceleration of hydrogen generation using seawater as a reactant” Int'l J. hydrogen Energy, 2016 ,1-10.
- Becker-Glad et al., Acid Acceleration of Hydrogen Generation Using Seawater as a Reactant, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.132.
- Kliphuis, J. et al., Study of Wake Generating System for an ASW Training Device, Department of the Navy Technical Report No. NAVTRADEVCEN 1297-1, (1984).
- Schlesinger, Herbert E. et al., Sodium Borohydride, Its Hydrolysis and its Use as a Reducing Agent and in the Generation of Hydrogen, J. Am. Chem. Soc. 75(1): 215-219.
Filed: Jan 25, 2017
Date of Patent: Jan 15, 2019
Patent Publication Number: 20180208462
Assignee: The United States of America as represented by the Secretary of the Navy (Washington, DC)
Inventors: Carol A. Becker (Del Mar, CA), Wayne E. Glad (Del Mar, CA), Brandon J. Wiedemeier (San Diego, CA)
Primary Examiner: Amber R Orlando
Assistant Examiner: Syed T Iqbal
Application Number: 15/415,659
International Classification: C01B 3/06 (20060101); H01M 8/065 (20160101); B01J 19/00 (20060101);